1. Field of the Invention
The present invention relates generally to modem apparatus for data communication systems and more particularly to an improved decision feedback phase lock loop for reducing decision errors by ignoring signals detected within certain predetermined erasure zones.
2. Description of the Prior Art
Decision feedback phase lock loops are commonly employed in data modems for recovering the transmitted carrier. See for example "A High Performance Digital QAM 9600 bit/s Modem" by Fumio Akashi et al, NEC Research and Development Journal No. 45, April 1977 pp. 38-49. A typical example of such phase lock loops is illustrated by the block diagram of FIG. 1 which includes a multiplier 10 for receiving a complex input signal and multiplying it by a feedback signal input 12 for demodulating purposes, a complex symbol detector 14 for detecting the particular constellation point received, a modulation removal phase detector 16 which compares the demodulated complex input signal to the complex data output signal and generates a signal proportional to the difference therebetween, a loop filter 18 which serves to smooth out differences between successive difference signals input thereto so as to ignore noise in such signals, and an exponential circuit 20 which converts the filtered difference signal into a complex form for feedback input to the multiplier 10 to accomplish demodulation. Note that (1) the input signal is in complex form, and (2) if the data modem is to operate over channels with phase jitter and phase hits, as commonly occurs, the phase lock loop bandwidth must be quite wide.
Although decision feedback loops have many advantages and are favored over other methods of carrier recovery, such as for example square-law type (or Costas-type) phase lock loops for biphase modulation, if the data detector does make an error, the phase detector will produce an incorrect phase adjustment which will tend to cause additional error. In other words, there is a tendency toward propagation of any errors that occur. This problem is most noticeable when data encoding constellations with a large number of elements are used in circuits having wide phase lock loop bandwidths. Since wide bandwidth is achieved by permitting the phase detector output to have a large effect on the phase estimate, when a decision error and resulting incorrect phase correction estimate occur, the latter can more easily adversely affect subsequent data decisions.